Embodiments of the inventive concept relate generally to memory technologies. More particularly, embodiments of the inventive concept relate to nonvolatile memory devices and nonvolatile memory systems.
Memory devices can be broadly classified into two groups based on whether they retain stored data when disconnected from power. These groups include volatile memory devices, which lose stored data when disconnected from power, and nonvolatile memory devices, which retain stored data when disconnected from power.
Examples of volatile memory devices include dynamic random access memory (DRAM), and static random access memory (SRAM), and examples of nonvolatile memory devices include electrically erasable and programmable read only memory (EEPROM), phase-change random access memory (PRAM), resistance random access memory (RRAM), and magnetic random access memory (MRAM).
EEPROM is one of the more common forms of nonvolatile memory in use today due to its ability to be efficiently programmed, read, and erased. Flash EEPROM (hereafter, “flash memory”), for instance, can be found in a wide range of modern electronic devices, including solid state drives, mobile phones, digital cameras, and many others.
Flash memory devices have different operating modes for performing program, read, and erase operations. These modes will be referred to as a program operation mode, a read-out operation mode, and an erase mode. The program and read-out operation modes typically operate on relatively small units of data, such as words or pages. In other words, data is typically programmed or read out in page or word units. The erase operation mode, on the other hand, typically operates on entire memory blocks, which comprise multiple pages. In other words, data is typically erased an entire memory block at a time.
Flash memory devices can be designed with various alternative memory cell array configurations according to different applications. The most common configurations are found in NAND-type flash memory devices, in which cell transistors are connected in series between a bitline and a ground line, and in NOR-type flash memory devices, in which cell transistors are connected in parallel.
In a flash memory device, memory cells often undergo several read-out operations while storing the same data. These repeated read-out operations can unintentionally change the stored data through a phenomenon referred to as soft programming. In soft programming, a relatively low voltage used to select the memory cells causes an unintended shift in the memory cells' respective threshold voltages. This shift, accumulated over many read operations, can lead to undesired changes in the stored data, referred to as a read-out disturbance. Moreover, the effects of soft programming become more significant as the features in a flash memory device become smaller and more densely integrated.